The present invention relates to a liquid crystal display device and a method for fabricating the liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device having a transmission display region and a reflection display region in each pixel, and a method for fabricating such a liquid crystal display device.
Due to the features of being thin and consuming low power, liquid crystal display devices have been used in a broad range of fields including office automation (OA) apparatuses such as wordprocessors and personal computers, portable information apparatuses such as portable electronic schedulers, and a camera-incorporated VCR provided with a liquid crystal monitor.
Such liquid crystal display devices include a liquid crystal display panel which does not emit light itself, unlike a CRT display and an electroluminescence (EL) display. Therefore, a so-called transmission type is often used as the liquid crystal display device, which includes an illuminator called a backlight disposed at the rear or one side thereof, so that the amount of the light from the backlight which passes through the liquid crystal panel is controlled by the liquid crystal panel in order to realize image display.
In such a transmission type liquid crystal display device, however, the backlight consumes 50% or more of the total power consumed by the liquid crystal display device. Providing the backlight therefore increases the power consumption.
In order to overcome the above problem, a reflection type liquid crystal display device has been used for portable information apparatuses which are often used outdoors or carried with the users. Such a reflection type liquid crystal display device is provided with a reflector formed on one of a pair of substrates in place of the backlight so that ambient light is reflected from the surface of the reflector.
Such a reflection type liquid crystal display device is operated in a display mode using a polarizing plate, such as a twisted nematic (TN) mode and a super twisted nematic (STN) mode which have been broadly used in the transmission type liquid crystal display devices. In recent years, there has been vigorous development of a phase change type guest-host mode which does not use a polarizing plate and thus realizes a brighter display.
The reflection type liquid crystal display device using the reflection of ambient light is disadvantageous in that the visibility of the display is extremely lower when the surrounding environment is dark. Conversely, the transmission type liquid crystal display device is disadvantageous when the environment is bright. That is, the color reproducibility is lower and the display is not sufficiently recognizable because the display light is less bright than the ambient light. In order to improve the display quality under a bright environment, the intensity of the light from the backlight needs to be increased. This increases the power consumption of the backlight and thus the resultant liquid crystal display device. Moreover, when the liquid crystal display device needs to be viewed at a position exposed to direct sunlight or direct illumination light, the display quality is inevitably lower due to the ambient light. For example, when a liquid crystal display screen fixed in a car or a display screen of a personal computer used at a fixed position receives direct sunlight or illumination light, surrounding images are mirrored, making it difficult to observe the display itself.
In order to overcome the above problems, a construction which realizes both a transmission mode display and a reflection mode display in one liquid crystal display device has been disclosed in, for example, Japanese Laid-Open Publication No. 7-333598. Such a liquid crystal display device uses a semi-transmissive reflection film which transmits part of light and reflects part of light.
FIG. 52 shows such a liquid crystal display device using a semi-transmissive reflection film. The liquid crystal display device includes polarizing plates 30a and 30b, a phase plate 31, a transparent substrate 32, black masks 33, a counter electrode 34, alignment films 35, a liquid crystal layer 36, metal-insulator-metal (MIM) elements 37, pixel electrodes 38, a light source 39, and a reflection film 40.
The pixel electrodes 38, which are the semi-transmissive reflection films, are extremely thin layers made of metal particles or layers having sporadical minute hole defects or concave defects therein formed over respective pixels. Pixel electrodes with this construction transmit light from the light source 39 and at the same time reflect light from outside such as natural light and indoor illumination light, so that both the transmission display function and the reflection display function are simultaneously realized.
The conventional liquid crystal display device shown in FIG. 52 has following problems. First, when an extremely thin layer of deposited metal particles is used as the semi-transmissive reflection film of each pixel, since the metal particles have a large absorption coefficient, the internal absorption of incident light is large and some of the light is absorbed without being used for display, thereby lowering the light utilization efficiency.
When a film having sporadical minute hole defects or concave defects therein is used as the pixel electrode 38 of each pixel, the structure of the film is too complicated to be easily controlled, requiring precise design conditions. Thus, it is difficult to fabricate the film having uniform characteristics. In other words, the reproducibility of the electrical or optical characteristics is so poor that control of the display quality in the above liquid crystal display device is extremely difficult.
For example, if thin film transistors (TFTs), which in recent years have been generally used as the switching elements of liquid crystal display devices, are attempted to be used for the above liquid crystal display device shown in FIG. 52, an electrode for the formation of a storage capacitor in each pixel needs to be formed by an electrode/interconnect material other than that for the pixel electrode. In this case, the pixel electrode made of the semi-transmissive reflection film, as in this conventional device, is not suitable for the formation of a storage capacitor. Moreover, even when the semi-transmissive reflection film as the pixel electrode is formed over part of the interconnects and elements via an insulating layer, the pixel electrode which includes a transmissive component hardly contributes to an increase in the numerical aperture. Also, if light is incident on a semiconductor layer of the switching element such as a MIM and a TFT, an optically pumped current is generated. The formation of the semi-transmissive reflection film as the light-shading layer is insufficient for the protection of the switching element from light. To ensure light-shading, another light-shading film is required to be disposed on the counter substrate.
The liquid crystal display device of this invention includes a first substrate, a second substrate, and a liquid crystal layer interposed between the first substrate and the second substrate, a plurality of pixel regions being defined by respective pairs of electrodes for applying a voltage to the liquid crystal layer, wherein each of the plurality of pixel regions includes a reflection region and a transmission region.
In one embodiment of the invention, the first substrate includes a reflection electrode region corresponding to the reflection region and a transmission electrode region corresponding to the transmission region.
In another embodiment of the invention, the reflection electrode region is higher than the transmission electrode region, forming a step on a surface of the first substrate, and thus a thickness of the liquid crystal layer in the reflection region is smaller than a thickness of the liquid crystal layer in the transmission region.
In still another embodiment of the invention, the occupation of an area of the reflection region in each of the pixel regions is in the range of about 10 to about 90%.
Alternatively, the liquid crystal display device of this invention includes a first substrate, a second substrate, and a liquid crystal layer interposed between the first substrate and the second substrate, wherein the first substrate includes: a plurality of gate lines; a plurality of source lines arranged to cross with the plurality of gate lines; a plurality of switching elements disposed in the vicinity of crossings of the plurality of gate lines and the plurality of source lines; and a plurality of pixel electrodes connected to the plurality of switching elements, the second substrate includes a counter electrode, a plurality of pixel regions are defined by the plurality of pixel electrodes, the counter electrode, and the liquid crystal layer interposed between the plurality of pixel electrodes and the counter electrode, and each of the plurality of pixel regions includes a reflection region and a transmission region.
In one embodiment of the invention, the first substrate includes a reflection electrode region corresponding to the reflection region and a transmission electrode region corresponding to the transmission region.
In another embodiment of the invention, the reflection electrode region is higher than the transmission electrode region, forming a step on a surface of the first substrate, and thus a thickness of the liquid crystal layer in the reflection region is smaller than a thickness of the liquid crystal layer in the transmission region.
In still another embodiment of the invention, the thickness of the liquid crystal layer in the reflection region is about a half of the thickness of the liquid crystal layer in the transmission region.
In still another embodiment of the invention, each of the pixel electrodes includes a reflection electrode in the reflection electrode region and a transmission electrode in the transmission electrode region.
In still another embodiment of the invention, the reflection electrode and the transmission electrode are electrically connected to each other.
In still another embodiment of the invention, each of the pixel electrodes includes a transmission electrode, and the reflection region includes the transmission electrode and a reflection layer isolated from the transmission electrode.
In still another embodiment of the invention, the reflection electrode regions overlap at least a portion of the plurality of gate lines, the plurality of source lines, and the plurality of switching elements.
In still another embodiment of the invention, at least either of the reflection electrode regions and the transmission electrode regions have a layer formed of the same material as a material for the plurality of gate lines or the plurality of source lines.
In still another embodiment of the invention, the occupation of an area of the reflection region in each of the pixel regions is in the range of about 10 to about 90%.
In still another embodiment of the invention, the first substrate further includes storage capacitor electrodes for forming storage capacitors with the pixel electrodes via an insulating film, wherein the reflection electrode regions overlap the storage capacitor electrodes.
In still another embodiment of the invention, the liquid crystal display device further includes microlenses on a surface of the first substrate opposite to the surface facing the liquid crystal layer.
In still another embodiment of the invention, each of the reflection electrode regions includes a metal layer and an interlayer insulating film formed under the metal layer.
In still another embodiment of the invention, the metal layer has a continuous wave shape.
In still another embodiment of the invention, a surface of the interlayer insulating layer is of a concave and convex shape.
In still another embodiment of the invention, the interlayer insulating layer is formed of a photosensitive polymer resin film.
In still another embodiment of the invention, the interlayer insulating layer covers at least a portion of either the switching element, the plurality of gate lines, or the plurality of source lines.
In still another embodiment of the invention, the reflection electrodes are formed at the same level as the plurality of gate lines or the plurality of source lines.
In still another embodiment of the invention, the reflection electrodes are formed at the same level as the plurality of gate lines, and the reflection electrodes are electrically connected to the gate lines for the pixel electrodes adjacent to the reflection electrodes.
In still another embodiment of the invention, the same signals applied to the counter electrode are applied to the reflection electrodes.
In still another embodiment of the invention, the reflection electrodes are formed at the same level as the plurality of gate lines, and the reflection electrodes form storage capacitors by overlapping drain electrodes of the switching elements or the transmission electrodes.
In still another embodiment of the invention, the reflection electrode is formed of A1 or an A1 alloy.
In still another embodiment of the invention, the transmission electrode is formed of ITO, and a metal layer interposes between the transmission electrode and the reflection electrode.
According to another aspect of the invention, a method for fabricating a liquid crystal display device is provided. The liquid crystal display device includes a first substrate, a second substrate, and a liquid crystal layer interposed between the first substrate and the second substrate, the first substrate including: a plurality of gate lines; a plurality of source lines arranged to cross with the plurality of gate lines; a plurality of switching elements disposed in the vicinity of crossings of the plurality of gate lines and the plurality of source lines; and a plurality of pixel electrodes connected to the plurality of switching elements, the second substrate including a counter electrode, a plurality of pixel regions are defined by the plurality of pixel electrodes, the counter electrode, and the liquid crystal layer interposed between the plurality of pixel electrodes and the counter electrode, each of the plurality of pixel regions including a reflection region and a transmission region. The method includes the steps of: forming the transmission electrode regions using a material having a high light transmittance on the first substrate; forming photosensitive polymer resin layers; and forming reflection layers made of a material having a high reflectance on the polymer resin layers.
In one embodiment of the invention, the photosensitive polymer resin layers have a plurality of concave and convex portions.
Alternatively, a method for fabricating a liquid crystal display device of this invention is provided. The liquid crystal display device includes a first substrate, a second substrate, and a liquid crystal layer interposed between the first substrate and the second substrate, the first substrate including: a plurality of gate lines, a plurality of source lines arranged to cross with the plurality of gate lines; a plurality of switching elements disposed in the vicinity of crossings of the plurality of gate lines and the plurality of source lines; and a plurality of pixel electrodes connected to the plurality of switching elements, the second substrate including a counter electrode, a plurality of pixel regions are defined by the plurality of pixel electrodes, the counter electrode, and the liquid crystal layer interposed between the plurality of pixel electrodes and the counter electrode, each of the plurality of pixel regions including a reflection region and a transmission region. The method includes the steps of: forming the transmission electrode regions using a material having a high light transmittance on the first substrate; forming protection films on the transmission electrode regions; and forming layers having a high reflectance on portions of the protection films to form the reflection electrode regions.
In one embodiment of the invention, the transmission electrode regions are formed at the same level as the plurality of source lines.
Thus, the invention described herein makes possible the advantages of (1) providing a liquid crystal display device of a type realizing both a transmission mode display and a reflection mode display simultaneously where the light utilization efficiencies of ambient light and light from a backlight are improved compared with the conventional liquid crystal display device of the same type and an excellent display quality is obtained, and (2) providing a method for fabricating such a liquid crystal display device. In particular, in the liquid crystal display device according to the present invention, the display quality obtained when the environment is bright significantly improves.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.